data class User(val id: Int, val name: String)
data class Book(val id: Int, val name: String)

class Service(private val myDatabase: CoroutineDatabase) {
    private val userCollection: CoroutineCollection<User> = myDatabase.getCollection("users")
    private val booksCollection: CoroutineCollection<Book> = myDatabase.getCollection("books")

    fun getUserById(id: Int): User? = userCollection.findOneById(id)
    fun getBookById(id: Int): Book? = booksCollection.findOneById(id)
    // ...
}

fun test() {
    val mongoDatabase = mockkCoroutineDatabase(relaxed = true)
    val usersCollection = mockkCoroutineCollection<User>(relaxed = true)
    val booksCollection = mockkCoroutineCollection<Book>(relaxed = true)
    val coroutineUsers: CoroutineCollection<User> = usersCollection.coroutine
    val coroutineBooks: CoroutineCollection<Book> = booksCollection.coroutine

    every {
        mongoDatabase.getCollection(any(), User::class.java)
    } returns usersCollection

    every {
        mongoDatabase.getCollection(any(), Book::class.java)
    } returns booksCollection

    val sut: Service = Service(mongoDatabase)
    

    val expectedUser = User(20, "Joe")

    // depending on the order I create the mocked collections this hangs or not
    coEvery {
        coroutineUsers.findOneById(any())
    } returns expectedUser

    assertEquals(expectedUser, sut.getUserById(20))
}

mockkCoroutineCollection

every { ofType<MongoCollection<T>>().coroutine } returns …